Crepe paper or tissue paper is used to produce various articles for household and domestic, professional and also industrial use. In particular, tissue paper is used to make toilet paper, kitchen paper and other products in roll or sheet form. In many applications, the tissue paper is subjected to a mechanical embossing process. The embossing process in substance consists in feeding a single or multiple ply of tissue paper through a nip defined between an embossing roller and a pressure roller. The embossing roller is provided with protuberances, which cooperate with the surface of the pressure roller. In some embodiments the embossing roller and the pressure roller are both made of hard material, such as steel, and are respectively provided with protuberances and cavities that mesh with each other.
In other more widespread embodiments, the embossing roller is provided with protuberances that penetrate an elastically yielding coating layer provided on the pressure roller, deforming it with respect to the substantially smooth cylindrical shape that this layer assumes when the pressure roller is at rest and not in contact with the embossing roller. The two rollers are pressed against each other so that the protuberances penetrate the lateral surface of the pressure roller as a result of the compressive deformation of the yielding coating of said pressure roller. The cellulose material, forming the web material, which passes through the nip formed between the two rollers, is permanently deformed with the formation of embossing protrusions having a pattern corresponding to that of the embossing protuberances of the embossing roller.
Embossing causes high mechanical stress and localized breakage of the cellulose fibers of the paper ply.
Embossing is used both for aesthetic reasons, in order to decorate the ply of paper, and above all for technical-functional reasons, for example in order to create, for example, mutual gluing areas between several plies forming a multiply web material. The glue is applied to the outer surfaces of the embossing protrusions so as to obtain application of the glue to limited areas. Embossing is also used to modify, alter or improve specific characteristics of the tissue paper, such as thickness, softness and absorption capacity.
Due to the high stress to which the cellulose fibers forming the ply are subjected in the areas in which the cellulose ply is deformed by embossing, the shapes of the embossing protuberances, their dimensions and their arrangement cannot be chosen at will, but rather specific restrictive criteria that impose constraints to the choice of the embossing pattern must be taken into account. Therefore, when producing a new embossing pattern it is always necessary to find a compromise between the technical-functional requirements, with which the pattern must comply, and the requirements of not subjecting the paper to excessive stress, as otherwise this could cause localized breakage or excessive weakening of the cellulose material.
The embossing rollers are engraved on their cylindrical surface to generate the embossing protuberances. Early engraving techniques involved mechanical machining, by chip removal. More modern engraving techniques are based on the use of laser and chemical etching. With embossing rollers engraved using old engraving techniques (for example with the formation of truncated pyramid shaped protuberances) the deformation of the ply of cellulose material that is obtained is uniformly distributed. Vice versa, weakening of the same ply is non-homogeneous, due to the arrangement of the cellulose fibers, which is not isotropic, as will be explained below.
With the current engraving techniques (by chemical etching and of variegated shape), the appearance of the embossing pattern is greatly improved, but the ply is no longer deformed uniformly; therefore, in addition to non-homogeneous weakening, this also causes non-homogeneous elongation of the ply, which leads to the formation of wrinkles, localized loosening and slippage of the ply being processed.
As known to those skilled in the art, tissue paper is normally produced with wet or water-based processes. A slurry of water and cellulose fibers with a dry percentage of less than 5% and typically in the order of 2-4% is produced in a refiner. After adding any necessary additives, for example moisture-resistant resins, dyes or the like, the slurry is distributed from headboxes onto a forming fabric or a forming felt. Through subsequent steps, the water is gradually drained from the layer of slurry formed on the forming fabric or felt to increase the percentage of dry matter inside the layer of slurry. After reaching a percentage of dry matter sufficient for the layer of cellulose material thus formed to have adequate mechanical strength, the layer of fibers is passed to drying means, such as a Yankee cylinder, a series of dry rollers or the like.
As a result of the method in which the fibers are distributed on the forming fabric, through a continuous flow of slurry from the linear nozzles of the headboxes, with a direction of feed concordant with the direction of feed of the forming fabric, although in the slurry the fibers assume a totally random orientation, once the slurry has been distributed on the forming fabric the majority of the fibers are arranged with a preferential orientation in machine direction, i.e. in the direction parallel to the direction of travel of the forming fabric. This ensures that the finished cellulose material has non-isotropic mechanical strength characteristics, and more specifically a lesser strength in cross direction and a greater strength in longitudinal direction, i.e. in machine direction. This poses further limitations to the shape and orientation of the embossing protuberances and of the embossing protrusions which will be generated on the cellulose material.
Normally, the protuberances and consequently the embossing protrusions on the paper have simple geometric shapes, typically truncated pyramid or truncated cone shapes. In some cases, embossing follows more complex patterns with embossing protuberances on the rollers and embossing protrusions on the paper having a linear trend, but nonetheless of limited length.
EP-A-0955157 discloses a method and a device for embossing and joining the plies to produce a multiple ply web material.
US-A-2006/0286885 and EPj-A-1708872 discloses embossed products having embossing protuberances with dot-shaped geometry, which have side surfaces with a different inclination on different sides of the protuberance.